Morphine-induced Place Preference Research Articles

Different roles of dopamine receptor subtypes in footshock stress-induced enhancement of morphine conditioned place preference

The present study investigated the involvement of dopamine mechanism in the effect of intermittent footshock stress on the morphine-induced place preference. A single intermittent footshock session significantly enhanced the place preference induced by 3.0 mg/kg morphine. This enhancing effect was inhibited by selective D 1 receptor antagonist SCH23390 and selective D 2 receptor antagonist sulpiride pretreatment 20 min before footshock session, suggesting dopamine D 1 and D 2 receptors are required for the development of intermittent footshock stress-induced enhancement of morphine-associated place preference. However, different from D 1 and D 2 receptors this enhancing effect was blocked by stimulation of dopamine D 3 receptor with selective D 3 receptor agonist 7-OH-DPAT pretreatment 20 min before footshock session which suggest dopamine D 3 receptor play a negative mediation effect on the intermittent footshock stress-induced this enhancement. These results indicate that dopamine D 1, D 2, and D 3 receptor subtypes play different roles in footshock stress-induced enhancement of morphine conditioned place preference.

Social stress is as effective as physical stress in reinstating morphine-induced place preference in mice

Relapse to drug-seeking in abstinent heroin addicts and reinstatement in experimental animals are observed when exposed to drug-associated stimuli or cues, the drug itself, and stressful events. It has been shown that footshock-induced stress increases the rewarding effects of opiates, delays extinction, and induces the reinstatement of drug-seeking. However, the effects of social stress on the reinstatement of opiate-seeking after extinction has not been studied. The role of physical (restraint and tail pinch) and social (social defeat) stressors on the reinstatement of morphine-induced conditioned place preference (CPP) was evaluated. Adult male OF1 mice were conditioned with 10, 20, or 40 mg/kg of morphine or saline. Only morphine-conditioned animals acquired CPP. All mice underwent extinction sessions until the CPP was extinguished. Then, the effects of physical or social stress on the reinstatement of CPP were evaluated. Morphine- and saline-conditioned animals were exposed to the respective stressor or control stress condition immediately or 15 min before reinstatement tests. In experiment 1, animals underwent restraint for 15 min. In experiment 2, animals were exposed to tail pinch or placed in a cage without any manipulation for 15 min. In experiment 3, animals performed an agonistic encounter with an isolated or anosmic mouse or were placed in a cage without any social contact or manipulation. Restraint, tail pinch, and social defeat in an agonistic encounter with an isolated mouse produce the reinstatement of CPP in morphine-conditioned animals. These data demonstrate that social stress is as effective as physical stress in reinstating morphine-seeking.

GABAA receptors of hippocampal CA1 regions are involved in the acquisition and expression of morphine-induced place preference

In the present study, the effects of bilateral intra-hippocampal CA1 (intra-CA1) injections of GABAA receptor agonist and/or antagonist on the acquisition and expression of morphine-induced place preference in male Wistar rats have been investigated. The conditioning treatments with subcutaneous (s.c.) injections of different doses of morphine (0.5–7.5 mg/kg) induced a conditioned place preference (CPP) for the drug-associated place in a dose-dependent manner. Intra-CA1 administration of the GABAA receptor agonist, muscimol (0.25, 0.5 and 1 μg/rat) significantly inhibited the morphine (5 mg/kg, s.c.)-induced CPP. Intra-CA1 injections of different doses of the GABAA receptor antagonist, bicuculline (0.25, 0.5 and 1 μg/rat), in combination with an ineffective dose of morphine (0.5 mg/kg, s.c.) elicited a significant CPP. However, muscimol or bicuculline by themselves did not elicit any effect on place conditioning. Furthermore, the muscimol-induced inhibition of morphine response was reversed by bicuculline (1 μg/rat, intra-CA1) administration. On the other hand, the bilateral intra-CA1 injections of muscimol (0.25, 0.5 and 1 μg/rat) or bicuculline (0.5, 1 and 2 μg/rat) significantly decreased the expression of morphine-induced CPP. Intra-CA1 administration of different doses of muscimol or bicuculline had no effect on locomotor activity in the testing phase. Our data indicated that the GABAA receptors of the hippocampal CA1 regions may play an important role in the acquisition and expression of morphine-induced place preference.

Involvement of nitric oxide system in enhancement of morphine-induced conditioned place preference by agmatine in male mice

Agmatine recently has been suggested as a neurotransmitter, is able to interact with various effects of morphine like analgesia and dependence. In this study, the effects of agmatine on rewarding properties of morphine, and the possible involvement of nitric oxide (NO) system has been evaluated in an unbiased conditioned place preference (CPP) paradigm. Agmatine (1, 5 and 10 mg/kg, i.p.) alone induced neither CPP nor conditioned place aversion (CPA). Morphine (0.01, 0.05, 0.1 and 0.5 mg/kg, s.c.), while unable to show CPP or CPA, induced CPP in mice pretreated with agmatine. L-arginine (200 mg/kg, i.p.), a NO precursor, significantly enhanced the effect of agmatine (5 mg/kg) on morphine (0.5 mg/kg)-induced place preference. N G-nitro- l-arginine methyl ester ( l-NAME; 2.5 mg/kg, i.p.), a non specific nitric oxide synthase (NOS) inhibitor, and aminoguanidine (50 and 100 mg/kg, i.p.), a specific inducible NOS inhibitor, significantly reduced the effect of agmatine (5 mg/kg) on morphine (0.5 mg/kg)-induced place preference. These results suggest the possible involvement of inducible nitric oxide system in potentiating effects of agmatine on morphine-induced place preference.

Repeated administration of dopaminergic agents in the nucleus accumbens and morphine-induced place preference

In the present study, the effects of repeated intra nucleus accumbens (intra-NAc) injections of dopamine receptor agents on morphine-induced conditioned place preference (CPP) in rats were investigated by using an unbiased 3-days schedule of place conditioning design. The animals receiving once daily subcutaneous (s.c.) injections of morphine (0.5–7.5 mg/kg) or saline (1.0 ml/kg, s.c.) showed a significant place preference in a dose-dependent manner. The maximum response was observed with 5 mg/kg of the opioid. Three days intra-NAc injections of apomorphine (0.5 and 1 μg/rat) followed by 5 days free of the drug, increased or decreased, respectively CPP induced by the lower dose of morphine (0.5 mg/kg, s.c.). Morphine-induced CPP was also significantly increased in the animals that had previously received the 3-days intra-NAc injections of SKF 38393 (4 and 8 μg/rat) or quinpirole (2 and 4 μg/rat, intra-NAc). The CPP induced by a higher dose of morphine (5 mg/kg, s.c.) was significantly decreased in the animals that had previously received the 3-days SCH 23390 (0.005 and 0.01 μg/rat; intra-NAc). On the other hand, the CPP induced by morphine (5 mg/kg, s.c.) was significantly increased in the animals that had previously received the 3-days sulpiride administration (5 μg/rat, intra-NAc). The 3-days administration of apomorphine, SKF 38393 or quinpirole, but not SCH 23390 and sulpiride reduced the locomotor activity in the test session. It is concluded that repeated injections of dopamine receptors agents followed by 5 days free of the drugs in the NAc can affect morphine reward.

Direct Involvement of Orexinergic Systems in the Activation of the Mesolimbic Dopamine Pathway and Related Behaviors Induced by Morphine

In this study, we investigated the role of orexinergic systems in dopamine-related behaviors induced by the mu-opioid receptor agonist morphine in rodents. Extensive coexpression of tyrosine hydroxylase with orexin receptors was observed in the mouse ventral tegmental area (VTA). The levels of dopamine and its major metabolites in the nucleus accumbens were markedly increased by the microinjection of orexin A and orexin B into the VTA. The subcutaneous morphine-induced place preference and hyperlocomotion observed in wild-type mice were abolished in mice that lacked the prepro-orexin gene. An intra-VTA injection of a selective orexin receptor antagonist SB334867A [1-(2-methylbenzoxazol-6-yl)-3-[1.5]naphthyridin-4-yl urea] significantly suppressed the morphine-induced place preference in rats. Furthermore, the increased level of dialysate dopamine produced by morphine in the mouse brain was significantly decreased by deletion of the prepro-orexin gene. These findings provide new evidence that orexin-containing neurons in the VTA are directly implicated in the rewarding effect and hyperlocomotion induced by morphine through activation of the mesolimbic dopamine pathway in rodents.

Intra-ventral pallidal glutamate antagonists block expression of morphine-induced place preference.

The role of ionotropic glutamate receptors within the ventral pallidum (VP) in the expression of conditioned place preference (CPP) and motor adaptations to morphine was evaluated. VP-cannulated rats were subjected to 3 days of conditioning in which saline was paired to one distinct chamber in the morning and morphine (8 mg/kg ip or its vehicle) was paired to an alternate chamber in the afternoon. This induced (a) CPP expression in drug-free rats 1 day later, which was blocked by immediate pretreatments with intra-VP injections of a glutamate antagonist cocktail (DL-2-amino-5- phosphonopentanoic acid lithium salt [AP-5] + 6-cyano-7-nitroquinoxaline-2,3-dione disodium salt [CNQX]), and (b) changes in motor function expressed following an acute morphine challenge 18 days later, which were absent if preceded by a 10-day treatment with the glutamate antagonists injected unilaterally once daily in alternating hemispheres. Thus, VP ionotropic glutamate receptors are critical mediators of the expression of place preference and motor adaptations subsequent to repeated morphine exposure.

P.1.c.009 Ethanol state-dependent learning: involvement of dopamine D2 receptors of dorsal hippocampus

In the present study, the effects of bilateral intra-hippocampal CA1 (intra-CA1) injections of GABAA receptor agonist and/or antagonist on the acquisition and expression of morphine-induced place preference in male Wistar rats have been investigated. The conditioning treatments with subcutaneous (s.c.) injections of different doses of morphine (0.5–7.5 mg/kg) induced a conditioned place preference (CPP) for the drug-associated place in a dose-dependent manner. Intra-CA1 administration of the GABAA receptor agonist, muscimol (0.25, 0.5 and 1 μg/rat) significantly inhibited the morphine (5 mg/kg, s.c.)-induced CPP. Intra-CA1 injections of different doses of the GABAA receptor antagonist, bicuculline (0.25, 0.5 and 1 μg/rat), in combination with an ineffective dose of morphine (0.5 mg/kg, s.c.) elicited a significant CPP. However, muscimol or bicuculline by themselves did not elicit any effect on place conditioning. Furthermore, the muscimol-induced inhibition of morphine response was reversed by bicuculline (1 μg/rat, intra-CA1) administration. On the other hand, the bilateral intra-CA1 injections of muscimol (0.25, 0.5 and 1 μg/rat) or bicuculline (0.5, 1 and 2 μg/rat) significantly decreased the expression of morphine-induced CPP. Intra-CA1 administration of different doses of muscimol or bicuculline had no effect on locomotor activity in the testing phase. Our data indicated that the GABAA receptors of the hippocampal CA1 regions may play an important role in the acquisition and expression of morphine-induced place preference.

Involvement of GABA B receptors of the dorsal hippocampus on the acquisition and expression of morphine-induced place preference in rats

In the present study, effects of intra-hippocampal CA1 (intra-CA1) injections of GABA B receptor agonist and antagonist on the acquisition and expression of morphine-induced place preference in male Wistar rats have been investigated. Subcutaneous administration of different doses of morphine sulphate (0.5–6 mg/kg) produced a dose-dependent conditioned place preference (CPP). Using a 3-day schedule of conditioning, it was found that the GABA B receptor agonist, baclofen (0.5–2 μg/rat; intra-CA1), or the GABA B receptor antagonist, phaclofen (1–3 μg/rat; intra-CA1), did not produce a significant place preference or place aversion. Intra-CA1 administration of baclofen (1 and 2 μg/rat; intra-CA1) decreased the acquisition of CPP induced by morphine (3 mg/kg; s.c.). On the other hand, intra-CA1 injection of phaclofen (1 and 2 μg/rat; intra-CA1) in combination with a lower dose of morphine (1 mg/kg) elicited a significant CPP. The response of baclofen (2 μg/rat; intra-CA1) was reversed by phaclofen (4 and 6 μg/rat; intra-CA1). Furthermore, intra-CA1 administration of baclofen but not phaclofen before testing significantly decreased the expression of morphine (3 mg/kg; s.c.)-induced place preference. Baclofen or phaclofen injections had no effects on locomotor activity on the testing sessions. It is concluded that the GABA B receptors in dorsal hippocampus may play an active role in morphine reward.

Agmatine Potentiates Morphine-Induced Conditioned Place Preference in Mice: Modulation by Alpha(2)-Adrenoceptors

The effects of agmatine, an endogenous polyamine metabolite formed by decarboxylation of L-arginine, and its combination with morphine on conditioned place preference (CPP) has been investigated in male mice. Our data show that subcutaneous administration of morphine (1-7.5 mg/kg) significantly increases the time spent in the drug-paired compartment in a dose-dependent manner. Intraperitoneal administration of agmatine (1-40 mg/kg) alone does not induce either CPP or conditioned place aversion, while combination of agmatine and subeffective doses of morphine leads to potent rewarding effects. Lower doses of morphine (0.1, 0.05, and 0.01 mg/kg) are able to induce CPP in mice pretreated with agmatine 1, 5, and 10 mg/kg, respectively. Concomitant intraperitoneal administration of UK 14 304 (0.5 mg/kg), a highly selective alpha2-agonist, with per se noneffective dose of morphine (0.5 mg/kg) and also its combination with noneffective doses of agmatine (1 mg/kg) plus morphine (0.05 mg/kg) produces significant CPP. UK 14 304 (0.05, 0.5 mg/kg) alone, or in combination with agmatine (1, 5 mg/kg) have had no effect. We have further investigated the possible involvement of the alpha2-adrenoceptors in the potentiating effect of agmatine on morphine-induced place preference. Selective alpha2-antagonists, yohimbine (0.005 mg/kg) and RX821002 (0.1, 0.5 mg/kg), block the CPP induced by concomitant administration of agmatine (5 mg/kg) and morphine (0.05 mg/kg). Yohimbine (0.001-0.05 mg/kg) or RX821002 (0.05-0.5 mg/kg) alone or in combination with morphine (0.05 mg/kg) or agmatine (5 mg/kg) fail to show any significant place preference or aversion. Our results indicate that pretreatment of animals with agmatine enhances the rewarding properties of morphine via a mechanism which may involve alpha2-adrenergic receptors.

Effects of extremely low-frequency electromagnetic fields on morphine-induced conditioned place preferences in rats

In the present study, we examined the effects of extremely low-frequency (ELF) electromagnetic fields on morphine-induced conditioned place preferences in rats. During the conditioning phase (12 days), three groups of rats were placed in a sensory cue-defined environment paired with morphine (10 mg/kg, i.p.) following exposure to either 20 Hz (1.80 mT) or 50 Hz (2.20 mT) or sham electromagnetic fields for 60 min/day, respectively, and were placed in another sensory cue-defined environment paired with physiological saline (1 ml/kg, i.p.) without exposure to electromagnetic fields. After finishing 12 days of conditioning, preference tests for the morphine-paired place were performed during a 10-day withdrawal period. The exposure to electromagnetic fields substantially potentiated morphine-induced place preferences in rodents, suggesting that ELF electromagnetic fields can increase the propensity for morphine-induced conditioned behaviors.

Role of the cholinergic system in the rat basolateral amygdala on morphine-induced conditioned place preference

The effects of intra-basolateral amygdala (intra-BLA) injections of physostigmine, atropine, nicotine and/or mecamylamine on morphine-induced conditioned place preference (CPP) in rats was investigated by using an unbiased 3-day schedule of place conditioning design. Animals that received 3 daily injections of morphine (0.5–10 mg/kg) subcutaneously (s.c.) or saline (1.0 ml/kg, s.c.) showed a significant preference for compartment paired with morphine. The maximum response was observed with 7.5 mg/kg of the opioid. Administration of the anticholinesterase drug, physostigmine (1, 3 and 5 μg/rat) with an ineffective dose of morphine (0.5 mg/kg) elicited a significant CPP. Injections of antimuscarinic receptor agent, atropine (1, 4 and 7 μg/rat) dose-dependently inhibited the morphine (7.5 mg/kg)-induced place preference. The injections of nicotine (0.75, 1 and 2 μg/rat) potentiated the morphine (0.5 mg/kg)-induced place preference, while the nicotinic receptor antagonist, mecamylamine (1, 3 and 6 μg/rat) dose-dependently inhibited the morphine (7.5 mg/kg)-induced place preference. Furthermore, administration of atropine (7 μg/rat) but not mecamylamine (6 μg/rat) reduced the response induced by different doses of physostigmine plus morphine. Moreover, mecamylamine (6 μg/rat) but not atropine (7 μg/rat) reduced the response induced by different doses of nicotine plus morphine. It is concluded that the muscarinic and nicotinic receptor mechanisms in the BLA may be involved in the acquisition of morphine-induced place preference.

Cross-reinstatement by cocaine and amphetamine of morphine-induced place preference in mice

The cross-reinstatement by psychostimulants of a conditioned place preference (CPP) induced by morphine was evaluated in mice. In Experiment 1, we examined the effects of a single dose of cocaine and amphetamine on a previously extinguished morphine CPP. After acquisition of CPP induced by morphine (40 mg/kg), animals underwent daily extinction sessions of 15 min duration until the CPP was extinguished. Subsequently, animals received a non-contingent injection of cocaine (25 mg/kg) or amphetamine (4 mg/kg), which produced the reinstatement of the extinguished morphine-induced CPP. In Experiment 2, we evaluated the reinstating effects of several priming doses of cocaine (Experiment 2A) or amphetamine (Experiment 2B). As in the first experiment, after conditioning with morphine (40 mg/kg), mice underwent daily 15 min extinction sessions. When the preference was no longer evident, we tested the effects of cocaine (0, 6.25, 12.5, 25 and 50 mg/kg) and amphetamine (0, 0.5, 1, 2 and 4 mg/kg) on the reinstatement of CPP. Doses from 12.5 mg/kg of cocaine upward and doses from 1 mg/kg of amphetamine upward effectively reinstated CPP. Our results demonstrate cross-reinstatement with psychostimulants and opiates, suggesting that in abstinent individuals, drug exposure can produce craving for the previously abused drug and relapse.

Repeated administration of dopaminergic agents in the dorsal hippocampus and morphine-induced place preference

The aim of the present experiments was to investigate whether repeated intra-hippocampal CA1 (intra-CA1) administration of dopaminergic agents can affect morphine-induced conditioned place preference (CPP). Effects of repeated intra-CA1 injections of dopamine (DA) receptor agonists and antagonists on morphine-induced CPP in rats were investigated using an unbiased 3-day schedule of place conditioning. Animals receiving once-daily subcutaneous (s.c.) injections of morphine (1-9 mg/kg) or saline (1.0 ml/kg, s.c.) showed a significant place preference in a dose-dependent manner: the maximum response was observed with 3 mg/kg morphine. Three days' intra-CA1 injections of apomorphine (0.25-1 microg/rat) followed by 5 days free of the drug, significantly decreased morphine CPP (1 and 3 mg/kg, s.c.). Moreover, pre-treatment with the highest dose of apomorphine (1 microg/rat) altered the effect of morphine to an aversive response. The morphine (1 and 3 mg/kg) CPP was also significantly decreased in animals that previously received three intra-CA1 injections of SKF 38393 (2-9 microg/rat), quinpirole (1-3 microg/rat) or sulpiride (1-3 microg/rat), and significantly increased in animals that had previously received three intra-CA1 injections of SCH 23390 (0.02 microg/rat). The 3-day pre-treatment with apomorphine, SKF 38393 or quinpirole reduced locomotor activity in the test session, while SCH 23390 and sulpiride did not have any influence on locomotor activity. It is concluded that repeated injections of DA receptor agents in the dorsal hippocampus, followed by 5 days free of the drugs, can affect morphine reward.

Effect of cyclosporin A on morphine-induced place conditioning in mice: involvement of nitric oxide

Cyclosporin A is shown to attenuate antinociceptive effects of morphine, development and expression of morphine-induced tolerance and dependency via nitric oxide (NO) pathway. In the present study, the effect of systemic cyclosporin A on morphine-induced conditioned place preference (CPP) and the probable involvement of nitric oxide were assessed in mice. Our data showed that administration of morphine (1, 2.5, 5, 7.5, 10 mg/kg) significantly increased the time spent in the drug-paired compartment in a dose-dependent manner. The maximum response was obtained with 5 mg/kg of morphine. Cyclosporin A (5, 10 mg/kg) and N G -nitro- l-arginine methyl ester ( l-NAME; 2.5, 5, 10 mg/kg), a nonselective nitric oxide synthase (NOS) inhibitor, did not induce either conditioned place preference or conditioned place aversion (CPA), while cyclosporin A (20 mg/kg) induced CPA. Both cyclosporin A (10, 20 mg/kg) and l-NAME (5, 10 mg/kg), in combination with morphine (5 mg/kg) during conditioning, significantly suppressed acquisition of morphine-induced place preference. Lower and per se noneffective doses of Cyclosporin A (1, 2.5, 5 mg/kg) and l-NAME (2.5 mg/kg), when coadministered, exerted a significant potentiating effect on the attenuation of morphine-induced place preference. Aminoguanidine (50, 100 mg/kg), the specific inducible nitric oxide synthase (iNOS) inhibitor, whether alone or in combination with cyclosporin A failed to show this inhibitory effect on morphine-induced place preference. In conclusion, decreasing nitric oxide production through inhibiting constitutive nitric oxide synthase may be a mechanism through which cyclosporin A attenuates morphine-induced place preference.

Role of extracellular signal-regulated kinase in the ventral tegmental area in the suppression of the morphine-induced rewarding effect in mice with sciatic nerve ligation.

We recently reported that micro-opioid receptor agonist morphine failed to induce its rewarding effects in rodents with sciatic nerve injury. In the present study, we investigated whether a state of neuropathic pain induced by sciatic nerve ligation could change the activities of the extracellular signal-regulated kinase (ERK) and p38 in the mouse lower midbrain area including the ventral tegmental area (VTA), and these changes could directly affect the development of the morphine-induced rewarding effect in mice. The sciatic nerve ligation caused a long-lasting and profound thermal hyperalgesia. A dose-dependent place preference induced by s.c. administration of morphine was observed in sham-operated mice, but not in sciatic nerve-ligated mice. We found here for the first time that nerve injury produces a sustained and significant reduction in protein levels of phosphorylated-ERK and -p38 in cytosolic preparations of the mouse lower midbrain. The inhibition of ERK activity by i.c.v. pre-treatment with either PD98059 or U0126 impaired the morphine-induced place preference. In contrast, i.c.v. treatment with a specific inhibitor of p38, SB203580, did not interfere with the morphine-induced rewarding effect. Immunohistochemical study showed a drastic reduction in phosphorylated-ERK immunoreactivity within tyrosine hydroxylase-positive cells of the VTA. These results suggest that a sustained reduction in the ERK-dependent signalling pathway in dopamine cells of the VTA may be implicated in the suppression of the morphine-induced rewarding effect under neuropathic pain.

Calmodulin inhibitor trifluoperazine attenuates the development and expression of morphine-induced conditioned place preference in rats

The effect of trifluoperazine, a calmodulin inhibitor, on morphine-induced conditioned place preference was examined in rats. Morphine (5, 10 mg/kg, i.p.) produced significant place preference for the drug-associated place. Trifluoperazine significantly suppressed the development as well as the expression of morphine-induced place preference in a dose-dependent manner, but it neither produced place preference or aversion, nor affected locomotor activity. Injection of 0.5 and 1.0 mg/kg apomorphine, a dopamine receptor agonist, did not alter the inhibition by trifluoperazine of morphine-induced place preference. Verapamil, at the dose that failed to change the place preference induced by morphine, enhanced the inhibition by trifluoperazine of morphine-induced place preference. These findings provide the first demonstration that trifluoperazine attenuates morphine-induced conditioned place preference in rats. The action of trifluoperazine might be produced through its inhibition of calmodulin, but is probably not related to dopamine receptor blockade.

Implication of spinal protein kinase C in the suppression of morphine-induced rewarding effect under a neuropathic pain-like state in mice

We previously demonstrated that spinal protein kinase C (PKC) is involved in the development of a neuropathic pain-like state induced by sciatic nerve ligation, and the morphine-induced rewarding effect is attenuated by sciatic nerve ligation in rodents. Here we first investigated whether sciatic nerve injury could change the activity of a conventional PKC (cPKC) and an atypical PKC isoform PKCζ in the mouse spinal cord. The second experiment was to investigate whether direct inhibition of spinal PKC by intrathecal (i.t.) administration of a specific PKC inhibitor, 2-{8-[(dimethylamino)methyl]-6,7,8,9-tetrahydropyrido[1,2-a]indol-3-yl}-3-(1-methyl-1 H-indole-3-yl)maleimide (RO-32-0432), could affect the rewarding effect induced by morphine following sciatic nerve ligation in mice. We found here that the activities of both cPKC and PKCζ in the spinal cord were clearly increased following sciatic nerve ligation. Furthermore, i.t. administration of RO-32-0432 reversed a long-lasting pain-like syndrome as indicated by thermal hyperalgesia following sciatic nerve ligation in mice. These data provide direct evidence that activated cPKC and PKCζ in the spinal cord may contribute to the development and maintenance of neuropathic pain. In the present study, we confirmed that the morphine-induced place preference was significantly suppressed by sciatic nerve ligation. It should be mentioned that i.t. pretreatment with RO-32-0432 significantly reversed the attenuation of morphine-induced rewarding effect following sciatic nerve ligation. These results suggest that activation of PKCs, including cPKC and PKCζ, within the spinal cord is directly responsible for the attenuation of the morphine-induced rewarding effect under a neuropathic pain-like state following sciatic nerve ligation in mice.

Role of the calcium/calmodulin-dependent protein kinase ii (CaMKII) in the morphine-induced pharmacological effects in the mouse

Calcium/calmodulin-dependent protein kinase II (CaMKII) is a family of multifunctional protein kinases that activates signaling pathways. The present study was designed to ascertain whether CaMKII could play a substantial role in the expression of morphine-induced antinociception, hyperlocomotion and rewarding effect in the mouse. An i.c.v. pretreatment with a CaMKII inhibitor KN-93 failed to affect the antinociception and hyperlocomotion induced by s.c. administration of a prototype μ-opioid receptor agonist morphine. In contrast, the morphine-induced place preference was significantly attenuated by i.c.v. pretreatment with KN-93. The levels of phosphorylated-CaMKII (p-CaMKII) in the limbic forebrain, but not in the frontal cortex and the lower midbrain, were significantly increased in morphine-conditioned mice, whereas the levels of CaMKII in three brain regions obtained from morphine-conditioned mice were not changed. This up-regulation of p-CaMKII in the limbic forebrain obtained from morphine-conditioned mice was significantly inhibited by i.c.v. pretreatment with KN-93. These results provide evidence that the increase in CaMKII activity in the mouse limbic forebrain may contribute to the rewarding effect, but not the antinociception and the hyperlocomotion, induced by morphine.

Involvement of Dopamine Receptors of the Dorsal Hippocampus on the Acquisition and Expression of Morphine-Induced Place Preference in Rats

In the present study, the effects of bilateral intrahippocampal CA1 injections of dopamine receptor agonists and antagonists on the acquisition and expression of morphine-induced place preference were examined in male Wistar rats. Subcutaneous administration of different doses of morphine sulphate (0.5-10 mg/kg) produced a conditioned place preference (CPP) dose-dependently. Using a 3-day schedule of conditioning, it was found that dopamine D1 receptor agonist, SKF 38393 (0.01-1 microg/rat), dopamine D1 receptor antagonist, SCH 23390 (0.25-1 microg/rat), dopamine D(2/3) receptor agonist, quinpirole (0.3-3 microg/rat) or dopamine D2 receptor antagonist, sulpiride (0.04-5 microg/rat) did not produce significant place preference. The administration of SKF 38393 (1 microg/rat) significantly potentiated the acquisition of morphine (0.5 and 2.5 mg/kg)-induced place preference. This potentiation was reversed by SCH 23390 (1 microg/rat) pretreatment. Quinpirole injection (0.3 microg/rat) induced CPP in combination with the lower doses of morphine but decreased the response of the higher doses of morphine. These responses of quinpirole were reversed by sulpiride (5 microg/rat) pretreatment. SCH 23390 or sulpiride reduced the acquisition of morphine (7.5 mg/kg)-induced place preference. The administration of sulpiride, but not other drugs, during acquisition showed an increase in the locomotor activity on the testing days. SKF 38393, SCH 23390 or sulpiride, but not quinpirole when used before testing, reduced the expression of morphine-induced place preference. Sulpiride, but not other drugs, increased locomotion when used before testing. It is concluded that dorsal hippocampal dopamine receptors may play an active role in morphine reward.